Danish and Swiss Scientists Split Electron Pairs from Superconductors Using Nanowires

After years of struggle and fierce international competition Danish and Swiss scientists succeeded in splitting electron pairs from superconductors using nanowires. Their sensational results were published in October in the scientific journal Nature.

Behind the data, together with researchers from the University of Basel in Switzerland, is Associate Professor Jesper Nygård, leader of the nanophysics section at the Nano-Science Center, Niels Bohr Institute, University of Copenhagen: "The results are very fundamental and are ground-breaking for new experiments in quantum mechanics. It has been a hard struggle for the last 10 years to reach this point and now it has finally succeeded", says a happy Jesper Nygård.

In superconductors the electric current flows without any resistance. It has been known for decades that the current is carried by electrons that are tightly bound in pairs. These electron pairs, so-called Cooper-pairs, can be compared to identical twins. In addition to looking alike, identical twins can have mental features in common. For electron pairs the similarity is even more extreme: even when they are physically separated, their fates are strongly entwined.

Schematic illustration of the Cooper-pair splitter.

Two electron twins from a superconductor (blue) are split at a T-junction and run in opposite directions in the nanowire (brown). They end in separate electrodes (green), whichmeasure an electronic current.

Credit: Nano-Science Center, Niels Bohr Institute, University of Copenhagen

By coupling a small superconductor to an ultrathin nanowire the researchers succeeded in separating the Cooper-pair from the superconductor. The two electrons run in opposite directions along the nanowire. The electrons each run in opposite directions at a nanoscale T-junction, but are still 'mentally connected' or 'entangled', as it is called in the jargon of quantum mechanics. Thus, the researchers can influence one of the electrons and see a reaction in the other, even though the pair is physically separated.

The split electron pair functions as a kind of building blocks in quantum mechanics. Therefore researchers now have the possibility of performing a series of new quantum mechanics experiments with the physically separated, but ”entangled” twin electrons. Nanowires, which researchers have used to split the Cooper-pairs, also hold great potential.

"Physicists have tested many different materials for the experiments, but in the end nanowires proved to be the most ideal material to couple with the superconductor. In Copenhagen we had a great deal of expertise in production of nanowires. Our contributions have therefore been the wires, while the measurements were carried out by the University of Basel", explains Jesper Nygård.

Since 2005 Ph.D. Claus B. Sørensen, Nano-Science Center, Niels Bohr Institute and Martin Aagesen, CEO of SunFlake A/S have worked to develop the cultivation of nanowires. The researchers use an advanced instrument, called MBE – Molecule Beam Epitaxy. The instrument is located at the Danish III-V Nanolab, which the Niels Bohr Institute and Technical University of Denmark run in collaboration.

Until just a few years ago no one knew the MBE instrument could create the extremely thin nanowires. Today the researchers use the Danish wires both in the firm SunFlake A/S for prototypes of new solar cells and in sensors for the medical industry.

The scientific article in Nature:  http://www.nature.com/nature/journal/v461/n7266/full/nature08432.html

Danish Scientists Improve Nanowire Manufacturing Techniques

Danish nanophysicists have developed a new method for manufacturing nanowires. The discovery has great potential applications in nanoelectronics and highly efficient solar cells.

It is PhD student Peter Krogstrup, Nano-Science Center, the Niels Bohr Institute at the University of Copenhagen, who developed the method during his dissertation.  "We have changed the recipe for producing nanowires. This means that we can produce nanowires that contain two different semiconductors, namely gallium indium arsenide and indium arsenide. It is a big breakthrough, because for first time on a nanoscale, we can combine the good characteristics of the two materials, thus gaining new possibilities for the electronics of the future", explains Peter Krogstrup.

Today only approximately 1 % of the world’s electricity comes from solar energy. This is because it is difficult to convert solar energy into electricity. It is a great advantage for the researchers to be able to combine different semiconductors in the same nanowire.  Different materials capture energy from the sun in different and quite specific absorption areas. When we manufacture nanowires of gallium indium arsenide and indium arsenide, which each have their own absorption area, they can collectively capture energy from a much wider area. We can therefore utilize more solar energy, if we produce nanowires from the two superconductors and use them for solar cells, explains Peter Krogstrup.

The nanowires of gallium indium arsenide and indium arsenide also have great potential in nanoelectronics. They can  be used in the new OLED displays and LEDs. But it requires sharp transitions between the two materials in the nanowire.

A nanowire made of the two semiconductors gallium indium arsenide (GaInAs) and indium arsenide (InAs) with gold (Au) as a catalyst. To the right a schematic illustration of the new cultivation method, where the semiconductor materials can move both from the top of the gold droplet and from the underside. Credit: University of Copenhagen

The cultivation of nanowires takes place in a vacuum chamber. The researchers lay a gold droplet on a thin disc comprising of the semiconductor and the nanowire grows up from below. In the transition between the two semiconductor materials in the gold droplet there was previously a mixing between the materials in the gold droplet and there was a soft transition between the materials. With the new method both of the materials can go from the top of the gold droplet or from the underside of the gold droplet. When the material comes from the underside, there is no mixing of the semiconductor materials. There is therefore a sharp transition on the atomic level between the gallium indium arsenide and indium arsenide.

- This sharp transition between the two semiconductors is necessary for the current – in the form of electrons, to be able to travel with high efficiency between the two materials. If the transition is soft, the electrons can easily get caught in the border area. The new mixed nanowire can be beneficial for many areas of nano research around the world, says Peter Krogstrup, who has been working at the Danish III-V Nanolab, operated in collaboration between the University of Copenhagen and the Technical University of Denmark.

A new collaboration between the company SunFlake A/S and The Danish National Advanced Technology Foundation has been kicked off this week. SunFlake A/S uses nanowires to develop prototypes of solar cells and they can also benefit from the new method in their continuing work. The nanophysicists’ discovery has just been published in the prestigious scientific journal Nano Letters.

PhD student Peter Krogstrup, Nano-Science Center (www.nano.ku.dk), Niels Bohr Instituttet (www.nbi.ku.dk), Københavns Universiet på mail: krogstrup@fys.ku.dk or mobile: +45 26 71 51 91

Communication Officer Gitte Frandsen, Nano-Science Center (www.nano.ku.dk), Københavns Universitet på mail: gf@nano.ku.dk or mobile: +45 28 75 04 58

Link to paper in Nano Letters: Junctions in Axial III−V Heterostructure Nanowires Obtained via an Interchange of Group III Elements, Nano Letters http://pubs.acs.org/doi/full/10.1021/nl901348d